1. Field of the Invention
This invention relates to an alkaline storage battery such as a nickel-metal hydride (Nixe2x80x94MH) storage battery, a nickel-cadmium (Nixe2x80x94Cd) storage battery, and nickel-zinc (Nixe2x80x94Zn) storage battery using nickel hydroxide as a positive electrode active material, and more particularly to an alkaline storage battery using nickel hydroxide active material containing a cobalt compound as a conductive auxiliary agent and a method of manufacturing it.
2. Description of the Related Art
In recent years, owing to rapid spreading of portable electronic/communication appliances, a high-capacity high-performance storage battery has been demanded more eagerly than before. Under such a circumstance, in the field of the alkaline storage battery using nickel hydroxide as a positive electrode active material, in order to facilitate the high performance of the storage battery, various proposals of improvement of the active material of a nickel positive electrode have been made. For example, it has been generally carried out to add a small quantity of the cobalt compound serving as a conductive auxiliary agent to nickel hydroxide which is a main component of the positive electrode.
However, only adding the cobalt compound serving as the conductive auxiliary agent cannot provide a high-capacity high-performance storage battery. Therefore, as a technique for accelerating the high capacity and high performance of the storage battery, the inventors of this invention have proposed in JP No. 2,589,123 an alkaline heat treatment method of coating a surface of nickel hydroxide with the cobalt compound and heating it within an atmosphere containing both alkaline solution and oxygen.
In the method proposed in JP No. 2,589,123, when the cobalt compound is heated in the atmosphere containing both alkaline solution and oxygen, highly conductive high-order (an average oxidation number of larger than +2)cobalt oxide is created so that the rate of using an active material is improved to realize the high-capacity and high-performance of the battery.
However, the presence of the high-order cobalt oxide on the surface of nickel hydroxide has presented a problem that the cobalt compound which does not directly participate in the reaction exists uniformly on the surface of the nickel hydroxide to impede contact of the nickel hydroxide with an electrolyte so that the high rate discharging characteristic attenuates.
Further, the high-order cobalt oxide created on the surface of nickel hydroxide is created in such a manner that a cobalt compound is first precipitated from a nucleus of a part of the compound on the surface of nickel hydroxide, and it is heat-treated in an atmosphere containing both alkali solution and oxygen. Therefore, the trace when the cobalt compound has been precipitated from a nucleus of a part of the compound on the surface remains as it is, which results in the high-order cobalt oxide. Accordingly, owing to the resultant attenuation of xe2x80x9csmoothxe2x80x9d, the electrode cannot be densely filled with the active material so that the battery with high capacity and high-performance cannot be realized.
This invention has been accomplished in order to overcome the inconvenience as described above.
An object of this invention is to provide an alkaline storage battery which provides high capacity and excellent high rate discharging characteristic by acquiring a nickel hydroxide active material which permits nickel hydroxide to be brought into direct contact with an electrolyte regardless with high-order cobalt oxide on the surface of nickel hydroxide.
In order to attain the above object, the alkaline storage battery according to this invention includes a positive electrode active material containing a nickel hydroxide compound as a main component. A part of the surface of the nickel hydroxide compound is unevenly covered with a high-order cobalt compound containing alkaline cations.
Since a part of the surface of the nickel hydroxide is unevenly covered with the high-order cobalt compound containing alkaline cations, the high-order cobalt compound with high conductivity produces a highly conductive network within the positive electrode so that the rate of using the active material is improved, thereby providing a high capacity storage battery. In this case, since only a part of the surface of the nickel hydroxide is unevenly covered with the cobalt compound so that it exposed, the nickel hydroxide not covered with the cobalt compound is brought into direct contact with the electrolyte, thus preventing the high rate discharging characteristic from being deteriorated. Thus, the alkaline storage battery with high capacity and excellent high rate discharging characteristic can be provided.
The method of manufacturing an alkaline storage battery includes: a covering step of covering a surface of a granulated nickel hydroxide compound containing the nickel hydroxide as a main component with a cobalt compound; an alkaline heat treatment step of alkaline-heat-treating the nickel hydroxide compound covered with the cobalt compound so that the cobalt compound is changed into a high-order cobalt compound containing alkaline cations; and an eluting step of making the nickel hydroxide compound covered with the high-order cobalt compound containing the alkaline cations in contact with or immersion in an acid water solution so that a part of the high-order cobalt compound is eluted.
In this way, when the surface of the nickel hydroxide compound is covered with the high-order cobalt compound and thereafter brought into contact with or immersed within the acid water solution, a part of the high-order cobalt compound on the outermost surface of the nickel hydroxide active material is eluted into the hydrochloric acid solution. Thus, the high-order cobalt compound containing alkaline cations is partially left on the surface of the nickel hydroxide compound. Using such an active material, a nickel electrode is made. Using this nickel electrode, an alkaline storage battery is manufactured. In this case, since the nickel hydroxide is brought into direct contact with an alkaline electrolyte with the battery, the high rate discharging characteristic can be improved.
In the covering step, the cobalt compound is precipitated from a nucleus on the surface of the nickel hydroxide compound. Further, in the alkaline heat treatment step, the cobalt compound is changed into the high-order cobalt compound. Therefore, upon completion of the alkaline heat treatment step, an uneven shape is formed on the nickel hydroxide compound is formed. Thereafter, the nickel hydroxide compound is brought into contact with or immersed in the acid water solution. In this case, the portion of the uneven shape is apt to be eluted so that it is smoothed. Thus, the nickel hydroxide compound with a part of the surface covered with the high-order cobalt compound containing alkaline cations can be densely filled into an electrode substrate, thereby realizing the high capacity of the battery.
Additionally, the covering step comprises a step of adding a water solution containing cobalt ions into a solution with granulated hydroxide nickel precipitated so that cobalt hydroxide is precipitated around a crystalline nucleus of a spherical hydroxide particle containing nickel hydroxide as a main component.
Further, the water solution containing the cobalt ions is preferably a water solution of cobalt sulfate.
Further, the covering step comprises a step of controlling pH of the solution so as to precipitates (segregate) the cobalt hydroxide. Preferably, pH of the solution is 9xcx9c10. This facilitates the coverage.
Further, preferably, the alkaline heat treatment step is to spray alkaline solution onto the nickel hydroxide compound covered with the cobalt compound within an atmosphere of oxygen.
Furthermore, preferably, the alkaline-heat-treatment is carried out at a temperature ranging from 50xc2x0 C. to 110xc2x0 C.
Moreover, preferably, the alkaline-heat-treatment step is to spray alkaline solution containing sodium hydroxide.
Incidentally, the acid water solution for eluting the high-order cobalt compound does not provides any particular difference in the elution effect as long as it is acid. However, it is desired to be hydrochloric acid solution, sulfuric acid solution, acetic acid solution, etc.
The above and other objects and features of this invention will be more apparent from the following description taken in conjunction with the accompanying drawings.